Swarming is a specialized form of bacterial motility that develops when cells that can swim are grown in a rich medium on the surface of moist agar. The cells become multinucleate, elongate, synthesize large numbers of flagella, excrete wetting agents, and advance across the surface in coordinated packs. Most studies of swarming have sought to define the developmental processes leading from the vegetative to the swarming state. Such studies ask why cells swarm. Our aim is to understand how cells swarm. The questions to be answered are pertinent not only to basic flagellar mechanics near a surface, but also to larger ramifications of this process, such as the group behavior of cells during surface colonization, including pattern generation and biofilm formation. If enough can be learned about what cells do, then it should be possible to accurately model the process and perhaps even interfere with it by novel means. The work will be done with Escherichia coli, the organism for which motility is best understood. 1) Using single and multi-color fluorescent labeling of flagellar filaments with pulsed laser illumination together with phase- contrast imaging, digital video recording, and frame-by-frame computer analysis, we will characterize the motion of cells and of their flagella near the leading edge of the swarm. How does the motion of flagella in the crowded environment of a swarm differ from that of cells swimming in dilute media? What roles do flagella play in coordinated behavior? 2) By tracking phase-contrast images, we will measure correlation distances and times of cells in different regions of the swarm. What is the range of coordinated motion? 3) How does the swarm boundary expand? If we can find suitable markers, we will try to learn how fluid moves in front of the leading edge of the swarm. Is fluid driven outward by flagellar motion or by osmotic flow from the underlying agar? It is our hope that by clarifying the behavior of the swarm cell, other researchers will have a more fundamental understanding to guide investigations on more complicated aspects of surface colonization, including invasiveness and pathogenicity. PUBLIC HEALTH RELEVANCE: We are trying to understand how flagellated bacteria move over moist surfaces, i.e., the mechanics of bacterial swarming. This motion enables cells to invade and colonize human tissue or medical devices, and thus is an important factor in pathogenesis.